3.1. Bond Strength
Three different failure modes of the splitting cylinder tensile test were observed during the test, as shown in Figure 3
. These three failure types are represented as Type A = pure interface failure, Type B = interface failure with partial substrate failure, and Type C = substrate failure. The results showed that the bonding for the surface roughened composite specimens were generally strong since most of the composite specimens failed in the NSC substrate.
Failure of composite samples exposed to 0% relative humidity occurred at the interface. This can be attributed to the fact that when new concrete (UHPC) was applied to a dry substrate concrete surface, part of its mixing water was absorbed into the substrate concrete and the cementitious materials in the UHPC directly in contact did not adhere firmly to the substrate concrete [23
The tensile strength of the splitting cylinder was calculated using Equation (1), and the values are shown in Figure 4
. The results showed the enhanced tensile performance of bulk UHPC, 5 times greater than bulk NSC, which is apparently due to the ‘‘bridging effect” of steel fiber in UHPC.
The general splitting tensile strength of the composite samples indicated an adequate strength level of bonding (>1.4 MPa), based on the quantitative bond strength quality proposed by Sprinkel and Ozyildirim [24
]. Generally, improved bonding performance between UHPC and substrate NSC regardless of the moisture content of the substrate can be attributed to the good workability of UHPC which enhanced its capability to fill the pores on the substrate surface [14
]. The good bond strength for the composite samples ensured that the ionic flow in the electrical migration test described next was related to the transport through the bulk material and cold-joint interface. No bond degradation or mechanical flaw was measured in the composite samples that would otherwise affect chloride transport.
The relatively small-sized samples did not capture well the influence of moisture on the bond strength, but some distinct behavior was observed between the samples with differential environmental moisture exposure conditions.
3.2. Chloride Penetration
shows the RCPT results in term of Total Charge Passed (TCP) in coulombs for the NSC substrate, UHPC, and the composite of NSC/UHPC with different substrate surface moisture content. The TCP could be related to the resistance of the concrete samples against chloride ion penetration. A lower TCP value indicated greater resistance to chloride ion penetration.
As shown in Figure 5
, the plain UHPC specimens had the lowest TCP values (i.e., 21 coulombs at 21 days), while the plain NSC specimen had TCP values more than 1100 coulombs after 21 days. As expected, the low bulk permeability of UHPC resulted in significantly lower chloride ion migration.
All the composite specimens, except the samples where substrate NSC had 0% RH before UHPC placement, exhibited TCP values of less than 750 coulombs after 21 days. This also seemed to signify that the conditioning environments for the NSC substrate (75%, 100% RH, and soaked) did not have a major influence on chloride permeability during the testing period. Adequate surface preparation with sufficient moisture levels apparently provided good bonding for all composite samples and similar performance to resist chloride ion penetration.
The high value of the recorded TCP for of the composite samples where the substrate had 0% RH before UHPC placement (70,000 coulombs) can be contributed to the capillary absorption of the NSC substrate, leading to quick absorption of chloride solution. Capillary absorption is very rapid and strong transport mechanisms compared to the other transport mechanisms.
Assuming that the flux of chloride transport is proportional to the surface area of the NSC (and little current passes through the UHPC component), the total ionic current passing through the composite concrete specimens would ideally result in a 1:2 ratio compared to the plain NSC samples. However, in the composite samples with 100% RH, the TCP values were larger. This could be an indication of preferential chloride penetration through the joint.
To assess the chloride penetration path through the joint interface, after splitting the samples under load, the substrate surfaces were sprayed with 1M AgNO3 solutions. The specimens were allowed to dry naturally at room temperature for 30 min. When a silver nitrate solution is sprayed on a concrete surface containing chloride ion, a photochemical reaction occurred. The chlorides bind with the silver to produce silver chloride (white precipitate). In the absence of chlorides, the silver instead bonds with the hydroxides present in the concrete and forms a brown precipitate of silver oxide [26
]. Representative photos after spraying AgNO3 indicator are shown in Figure 6
. NT Build 492 [22
] recommends utilizing a suitable ruler to measure the penetration depths after the chloride migration test. Based on that, seven depths were measured, and an average measurement is shown in Table 3
Chloride penetration could be readily observed by the differentiation in surface coloration on the NSC portion of the specimens, and bulk chloride penetration was generally seen in the upper portion of the specimens. No chloride penetration could be captured for UHPC samples mainly due to its remarkable impermeability. A generally uniform chloride penetration front through the joint developed in the composite concrete specimens conditioned in 0%, 75% RH, and soaked in water. As expected, the apparent bulk chloride penetration depth through the joint was higher with the presence of excess moisture levels. The highest chloride penetration depth (up to 76 mm) was measured for samples soaked in water. Generally, lower chloride penetration depths (up to 16 mm) were measured for the other cases.
However, it was apparent that chloride ions can also penetrate along the surface of the joint interface. For the specimens conditioned at 0%RH, the high measured TCP values evidently occurred along the edge of the specimen. The specimens conditioned at 100%RH also showed indication of non-uniform chloride penetration along the surface of the cold joint, as evidenced by localized regions of silver chloride penetrations throughout the joint surface. In part to address the possible means for non-uniform chloride penetration, the bulk chloride transport was compared to the bond of the cold joint.
As shown in Figure 7
, The specimens soaked in water showed both the largest bulk chloride penetration and the highest split tensile strength which would indicate that bulk diffusion was prominent here. The specimens conditioned at 100%RH showed the lowest bulk chloride penetration and the lowest split tensile strength. this indicates that the joint environment there did not provide strong resistance to chloride penetration and as a result non-Fickian transport can occur. Similar behavior would be expected for the other samples.
The test results give an indication that chloride transport through the cold joint strongly depends on the level of available moisture in the concrete. Higher water levels allow for better hydration of the repair material, especially as UHPC has inherently low water content and may develop conditions for self-desiccation. The better cement hydration would increase resistance to chloride penetration.